1. Field of the Invention
The present invention relates to an image pickup apparatus for performing digital signal processing.
2. Related Background Art
With the progress of semiconductor technologies, image pickup apparatuses using a CCD as a solid state image sensor have been decreased in size and weight and improved in performance. In particular, with the development of digital technologies a large number of image pickup apparatuses have been proposed recently, in which most internal signal processing of the apparatus is done by digital signal processing using a digital IC.
In these apparatuses, a signal portion is extracted from an output signal of a CCD acting as an image sensor by using a sample-and-hold circuit. After the gain of the signal portion is changed by a variable gain amplifier, the signal is converted into a digital signal by an A/D converter. The resulting digital signal is applied to the digital IC described where the signal is subjected to digital signal processing. The processed signal is D/A-converted and output to a VTR or a television monitor. Alternatively, the signal is output to external equipment as a digital video signal without being D/A-converted.
In these conventional apparatuses, since the CCD output signal is weak and fast, the sample-and-hold circuit uses a bipolar process IC having a high S/N ratio and good frequency characteristics. The variable gain amplifier uses a bipolar process IC which has good frequency characteristics, reduces low-frequency noise, and causes little variation in the manufacture. Also, the AID converter uses a CMOS analog process IC in order to reduce the consumption power.
Unfortunately, in these conventional image pickup apparatuses the weak output signal from the CCD is sometimes affected by noise generated by the digital processing circuit, resulting in a low S/N ratio. If the noise is low-frequency noise, it is possible to reduce it by double correlation sampling. However, in this case a high-speed operation is necessary to improve the noise reduction performance, and this further increases the consumption power and also requires a high accuracy. The result is a problem of characteristic variations in the manufacture, and consequently the yield is decreased or an adjustment circuit is required.
The connecting portion between, e.g., the sample-and-hold circuit and the variable gain amplifier for processing analog signals is also readily influenced by the noise from the digital processing circuit. Furthermore, as a problem of the whole image pickup apparatus, in placing the analog processing circuit and the digital processing circuit inside a small apparatus, it is necessary to form an electromagnetic shield in order to remove the mutual influence, or the apparatus cannot be miniaturized to a necessary size in order to prevent the noise influence. Also, the S/N ratio is lowered.
Especially when analog signals are transferred from the sample-and-hold circuit to the variable gain amplifier and from the variable gain amplifier to the AID converter, the S/N ratio is gradually lowered due to variations in the temperature resulting from active elements of the individual circuits, low-frequency noise, external noise, or power supply noise. To prevent this, the characteristics of each individual circuit must be extremely improved, with the result that the circuit configuration is complicated and the consumption current is increased. It is also necessary to add a compensation circuit for a voltage drift caused by a temperature change or to add an adjustment circuit. Furthermore, if the whole apparatus is electromagnetically shielded to prevent noise from the outside of the apparatus from affecting the analog signals, the apparatus is increased in size and weight.
Moreover, it is necessary to use bipolar process ICs in the sample-and-hold circuit and the variable gain amplifier, since the accuracy required of ICs in these circuits is very high. This increases the consumption power and decreases the degree of integration.